In one of the commercially applied methods for manufacturing titanium dioxide pigment particles, the so-called chloride process, titanium tetrachloride (TiCl4) is reacted with an oxidising gas, such as oxygen, air, etc., and with specific additives in a tubular reactor to obtain titanium dioxide and chlorine gas:TiCl4+O2→TiO2+2Cl2 
The TiO2 particles are subsequently separated from the chlorine gas. Known additives are AlCl3 as a rutiliser, and steam or alkali salts as nucleating agents.
This process is usually performed in a single stage, as described in U.S. Pat. No. 3,615,202 or EP 0 427 878 B1, for example. This kind of reaction is, however, energetically unsatisfactory because, owing to the high activation energy of TiCl4 oxidation, the educts have to be preheated to such a degree that an adiabatic mixed temperature of the educts of approx. 800° C. is reached before the onset of the reaction in order for the reaction to take place completely. However, the oxidation reaction is highly exothermal, meaning that, following complete, adiabatic conversion, the temperature of the product stream is roughly 900° C. higher than that of the educts. Before the TiO2 particles are separated from the gaseous reaction products with the help of a filter, this mixture has to be substantially cooled in a cooling section in order to avoid damage to the filter.
For the purpose of energetic optimisation, multi-stage versions of the chloride process have therefore been developed, in which only part of the educts is heated and added to a first stage. The rest of the educts is added to the second stage after just slight heating, or even without heating. There, the educts are heated by the reaction enthalpy released in the first stage, in turn reacting themselves. TiCl4 alone or TiCl4 and oxygen can be added to the second stage. In addition to a second stage, it is furthermore possible to provide further stages using slightly heated or cold educts.
For example, EP 0 583 063 B1 describes the two-stage introduction of TiCl4 into the reactor. TiCl4 with a temperature of at least 450° C. and mixed with AlCl3 is fed into the hot oxygen stream at a first inlet, and with a temperature of 350° C. to 400° C. and without AlCl3 at a further inlet.
The method according to EP 0 852 568 B1 makes provision for not only the TiCl4 to be added in two stages, but also the oxygen. The objective of this method is effective control of the mean TiO2 particle size, and thus of the tone of the TiO2 pigment base material. In this case, TiCl4 vapour with a temperature of roughly 400° C. is initially introduced into an oxygen stream with a temperature of roughly 950° C. Formation of the TiO2 particles and particle growth take place in the downstream reaction zone. Less highly heated TiCl4 vapour (approx. 180° C.) is added at a second inlet. Oxygen with a temperature between 25° C. and 1,040° C. is introduced at the second inlet, the temperature of the mixture being sufficient to initiate the reaction.
The multi-stage method according to U.S. Pat. No. 6,387,347 B1 is additionally designed to reduce the formation of agglomerates. To this end, the previously heated TiCl4 stream is divided into two split streams before addition to the reactor. One split stream (about 60%) is oxidised in the first stage of the reactor, the second split stream (about 40%) being cooled (de-superheated) by injecting liquid TiCl4 and then added to the reactor. De-superheating takes place outside the reactor, where the temperature does not drop below the condensation temperature of the overall stream.
A similar method for manufacturing TiO2 is described by US 2008/0075654 A1. This patent application comprises the technical teaching that the particle size of the TiO2 product may be decreased by lowering the entrance temperature of the second TiCl4 split stream. The effect is increased if the entrance temperature of the second TiCl4 split stream is lower than the entrance temperature of the first TiCl4 split stream and the effect is decreased if the temperature ratio is other way around (see examples 1 and 4).
US 2007/0172414 A1 discloses a multi-stage method for reacting TiCl4 and O2, where gaseous TiCl4 is fed into the reactor in the first stage, and liquid TiCl4 in the second stage. This method permits energy savings and improvement of the particle size range.
The common feature of all these processes is that the educts fed into the first stage are highly heated. The first stage is thus operated with highly heated oxygen and heated, vaporous TiCl4. One disadvantage of this form of the multi-stage reaction is, however, that the mean particle size increases with the proportion of educts in the second and subsequent stages. This effect can probably be explained as follows: two competing reaction pathways are possible when reacting TiCl4 and oxygen. On the one hand, TiCl4 and O2 can react with each other directly in the gas phase (homogeneous gas-phase reaction), as a result of which TiO2 molecules are formed that grow into particles by colliding and sintering with each other. On the other hand, TiCl4 can attach itself to the surface of existing TiO2 particles and react with oxygen there to form TiO2. This second reaction pathway does not lead to the formation of new particles, but to an increase in the size of existing particles (surface reaction).
The former mechanism is favoured in the single-stage oxidation reaction, because virtually no particles are present at the time of reaction of the TiCl4 and the O2. In the two-stage and multi-stage reaction, however, unburned TiCl4 is added to a stream of TiO2 particles, meaning that the reaction mechanism shifts in favour of the surface reaction in comparison with the single-stage reaction. The result is an increase in the mean particle size. The velocity of the surface reaction may be decreased so that the mean particle size increases to a lesser degree by lowering the temperature of the reactants (TiCl4, O2) in one split stream by de-superheating—as described in U.S. Pat. No. 6,387,347 B1 and US 2008/0075654 A1. Yet, in the end the increase of the mean particle size must be counteracted by increased addition of KCl or another growth inhibitor. However, these inhibitors are highly corrosive, resulting in increased corrosion of the equipment and a greater maintenance effort.